Referring to FIGS. 1(a)-1(d), one known method of bonding a layer of diamond to a silicon wafer to produce a diamond on silicon structure is shown. The method starts with a composite wafer 10 having a layer of CVD diamond 12 deposited on a layer of epitaxially grown silicon 14 on a device wafer layer 16, as illustrated in FIG. 1(a). Next a layer of poly-silicon 18 is deposited on the CVD diamond layer 12, as shown in FIG. 1(b). Once the layer of poly-silicon 18 is deposited, a bonding layer 20 is formed when the poly-silicon layer 18 and a handle wafer layer 22 are bonded together, as shown in FIG. 1(c). This layer bonding is accomplished through well known wafer bonding methods, such as the methods disclosed in U.S. patent application Ser. No. 07/921,197, filed Jul. 28, 1992, assigned to the assignee of the present invention and incorporated herein by reference, U.S. patent application Ser. No. 07/939,786, filed Sep. 3, 1992, assigned to the assignee of the present invention and incorporated herein by reference, and U.S. patent application Ser. No. 08/029,860, filed Mar. 11, 1993, assigned to the assignee of the present invention and incorporated herein by reference. Finally, the composite wafer 10 is flipped over and the device wafer layer 16 is thinned, as shown in FIG. 1(d).
The method described in FIGS. 1(a)-1(d) has several problems. The epitaxially grown silicon layer 14 is exposed to the diamond deposition and the poly-silicon deposition which can damage the silicon layer 14, as well as affect the distribution of intended dopants, such as boron and phosphorous, in the silicon layer 14. Additionally, there is a sharp interface between the diamond layer 12 and the poly-silicon layer 18 which results in only limited chemical bonding and, therefore, poor adhesion between diamond layer 12 and poly-silicon layer 18. This can increase the likelihood of film delamination and stress between the two layers. There is also a poor thermal match between the thermal coefficients of expansion for diamond layer 12 and the poly-silicon layer 18. This abrupt compositional change can result in warpage of the composite wafer 10 at elevated processing temperatures due to mismatched expansion and contraction of the layers. Further, the steps in forming the composite wafer 10 cannot be completed in one continuous process, as the poly-silicon layer is deposited separately from the diamond layer. This requires separate processing steps which adds to the manufacturing costs.
Referring to FIGS. 2(a)-2(h), another known method of bonding a layer of diamond to a silicon wafer to produce a silicon on diamond structure is shown. The method be:gins with a composite wafer 24 having a diamond layer 26 deposited on a substrate wafer layer 28, as shown in FIG. 2(a). Next a first poly-silicon layer 30 is deposited on the diamond layer 26, as shown in FIG. 2(b). Once the layer of poly-silicon 30 is deposited, a bonding layer 32 is formed when the poly-silicon layer 30 and a silicon handle wafer layer 34 are bonded together, as shown in FIG. 2(c). This wafer bonding is accomplished through well known wafer bonding methods, such as the methods disclosed in U.S. patent application Ser. No. 07/921,197, filed Jul. 28, 1992, assigned to the assignee of the present invention and incorporated herein by reference, U.S. patent application Ser. No. 07/939,786, filed Sep. 3, 1992, assigned to the assignee of the present invention and incorporated herein by reference, and U.S. patent application Ser. No. 08/029,860, filed Mar. 11, 1993, assigned to the assignee of the present invention and incorporated herein by reference. In the next step, the substrate wafer layer 28 is removed from the CVD diamond layer 26, as shown in FIG. 2(d) and the composite wafer 24 is flipped-over, as shown in FIG. 2(e). A second poly-silicon layer 36 is deposited on the CVD diamond layer 26 as shown in FIG. 2(f). A bonding layer 39 is formed between device wafer layer 38 and poly-silicon layer 36 when device wafer 38 is bonded to poly-silicon layer 36, as shown in FIG. 2(g). This wafer bonding is again accomplished through well known wafer bonding methods, such as the methods disclosed in U.S. patent application Ser. No. 07/921,197, filed Jul. 28, 1992, assigned to the assignee of the present invention and incorporated herein by reference, U.S. patent application Ser. No. 07/939,786, filed Sep. 3, 1992, assigned to the assignee of the present invention and incorporated herein by reference, and U.S. patent application Ser. No. 08/029,860, filed Mar. 11, 1993, assigned to the assignee of the present invention and incorporated herein by reference. Finally, the remaining first poly-silicon layer 30, bonding layer 32 and handle wafer layer 34 are removed from the CVD diamond layer 26, as shown in FIG. 2(h). Device wafer 38 can then be thinned to the appropriate thickness.
The method shown in FIGS. 2(a)-2(h) has the same problems as those discussed above for the method shown in FIGS. 1(a)-1(d). Additionally, with this method the diamond layer backside can be damaged from exposure to high ambient temperatures during high temperature processing. The prior art methods and techniques thus require bonding the diamond layer or composite diamond and silicon structure to substrate and handle wafers in order to form diamond on silicon wafers and devices.
It is a general object of the present invention to provide an improved method for bonding layers.
It is another object of the present invention to provide an improved method for depositing poly-silicon on diamond.
It is another object to provide one or more reliable transition layers for bonding CVD diamond on silicon wafers to substrate and handling wafers.